Chiang Mai Journal of Science

Energetically stable AlAs diamondoids are used as building blocks to investigate AlAs nanocrystal and bulk properties using density functional theory at the local spin density approximation (LSDA) with 6-31+G** basis states that include diffusion and polarization functions. Energy gap of AlAs diamondoids vary according to confinement theory with shape fluctuations. AlAs diamondoids vibrational force constant reach 0.85 mDyne/ which is less than that of bulk tin. Al-As hexamantane vibrational longitudinal optical (LO) mode frequencies and reduced masses reach 381.9 cm-1 and 32.3 amu respectively. Size variation of UV-Vis shows that the maximum optical peak moves from approximately 8 eV to nearly 3.2 eV as the size of AlAs diamondoids and molecules increases. NMR spectra of AlAs diamondoids are analyzed as function of diamondoids size. 1H-NMR shielding of AlAs diamondoids show values that are split in which Al-H shielding is lower than As-H shielding. Natural bond orbital (NBO) population analysis shows that present diamondoids bonding differ from ideal sp3 bonding expected for diamond and zincblende structures. The bonding for AlAs electronic orbitals at the center of AlAs octamantane is Al[core]3s0.783p0.713d0.024p0.125p0.01 As[core]4s1.524p4.744d0.03. The electronic occupation strongly depends on the distance between Al and As atoms and diamondoid surface.